Photo Conductivity on Photo Current

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<p>Journal of The Electrochemical Society, 153 1 A1-A4 20060013-4651/2005/153 1 /A1/4/$20.00 The Electrochemical Society, Inc.</p> <p>A1</p> <p>Effects of Bulk Photoconductivity on Photocurrent Action Spectra of Molecular p-n Heterojunction Solar CellsOleg Shevaleevskiy,a,b,z Liudmila Larina,b Seung Yeop Myong,a and Koeng Su Limaa b</p> <p>Department of Electrical Engineering and Computer Science, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea Institute of Biochemical Physics RAS, 119991 Moscow, Russia</p> <p>We have investigated how bulk photoconductivity inuences the photovoltaic parameters of zincphthalocyanine-fullerene ZnPc/C60 p-n heterojunction solar cells. The results indicate that the photocurrent action spectrum of a cell depends strongly on the photoconductivity and spectral characteristics of each component material. We therefore propose a model that simulates the action spectra of the short-circuit photocurrent in the molecular organic solar cells, and our model is based on the assumption that the photocurrent action spectrum depends on the bulk layer photoconductivity. 2005 The Electrochemical Society. DOI: 10.1149/1.2126576 All rights reserved. Manuscript submitted February 26, 2005; revised manuscript received June 30, 2005. Available electronically November 2, 2005.</p> <p>Molecular organic photovoltaic devices based on p-type semiconductors, such as metal-phthalocyanines MPc where M = Zn, Cu, TiO, or H2 , have recently attracted great interest due to the appearance of n-type fullerenes C60 , which form photosensitive p-n junctions with MPc.1-4 The efciency of organic solar cells is still lower compared to traditional solid-state devices.5 Initially, a relatively large power conversion efciency for a p-n junction organic photovoltaic cell based on p-type CuPc and n-type perylene derivative was realized by Tang.6 Fullerenes were successfully applied in different constructions of molecular photovoltaic cells for fabricating p-n heterojunctions,1,2 improvement of organic cell performance by doping,7,8 and for manufacturing plastic-type solar cells.9 Recently, a record efciency of around 4% was reported for p-i-n multilayer devices that used ZnPc and C60 modied molecular layers.10 However, the power conversion efciency of any molecular photovoltaic device is limited by poor charge carrier mobility 1 cm2 /V s and small exciton diffusion length LD 10 nm in organic layers.11,12 In condensed phthalocyanine layers, exciton diffusion length may depend on the fabrication conditions, and the reported LD values vary from 10 to 30 nm for ZnPc7,13 and from 12 to 60 nm for CuPc lms.14,15 The photoexcitations created in the bulk of a thick molecular layer do not reach the interface region of the p-n junction and therefore do not contribute to the photocurrent of the device. Thus, the carrier production regions are limited to the dimension of space-charge depletion layers that only extend over the lm interfaces. Due to this the light illumination produces two kinds of photoeffects: photoconductivity in the bulk of the layer that decreases the cell serious resistance, and a photovoltaic effect in the interface region resulting in the generation of the photocurrent. Since Simon and Andre16 reported signicant information about photogenerated carriers at the p-n heterojunction, various models have been proposed that simulated the photocurrent of organic molecular solar cells.17-20 However, the role of bulk layer photoconductivity is still being debated. We assume that the spectral dependence of series resistance in molecular organic cells should not be neglected because of its inuence on the cell photocurrent. Normally, the thickness of a bulk layer in organic solar cells several times exceeds the dimension of a depletion layer. Hence, the series resistance can severely limit the shape of photocurrent action spectrum and the conversion efciency. To prove this supposition, we report on the performance of fabricated n-C60 /p-ZnPc solar cells. We also propose a theoretical model that describes the behavior of photocurrent action spectra and effects of the bulk layer photoconductivity.</p> <p>Experimental Thin molecular layers were vacuum sublimated at a pressure of 106 Torr from resistively heated quartz crucibles using conventional ZnPc and C60 99.9% purity powder purchased from Kodak and MER Corp., respectively. Before the deposition, ZnPc powder was predominantly recrystallized under argon ambient by train sublimation. Figure 1 shows the arrangement of the fabricated p-n heterojunction solar cell produced by successive evaporation of C60 and ZnPc. First, we deposited n-C60 layers on Corning glass substrates coated with indium tin oxide ITO transparent front electrodes. After deposition, the samples were kept under an ultrahigh vacuum of 107 Torr during 24 h to reduce the initial oxygen content. Then, without exposure to air, the p-ZnPc layers were successively evaporated. Finally, the coplanar gold Au back contacts with a thickness of 50nm and an area of 2 10 mm were deposited on top of the device. Thus, we fabricated photovoltaic cells with a structure of ITO/n-C60 /p-ZnPc/Au. To provide a comparative study of organic layer thickness on cell performance, we have prepared three types of cells with the following parameters: A n-C60 80 nm /p-ZnPc 120 nm , B nC60 400 nm /p-ZnPc 400 nm , and C n-C60 800 nm /pZnPc 800 nm . With the same deposition conditions, single layers of n-C60 and p-ZnPc were prepared on ITO substrates for the UVvisible UV-vis optical spectra and photoconductivity measurements. Dark and photocurrent of single layers have been measured at constant dc electrical eld 105 V/cm in sandwich cell conguration between ITO front and Au back electrodes. Action spectra of short-circuit photocurrent for C60 /ZnPc cells were recorded under illumination from the ITO side. While measuring the photocurrent, we appropriately ltered and calibrated irradiation from a 500W xenon lamp through a monochromator to obtain a constant incident photon intensity PIN of 10 W/cm2 in the wavelengths of 360 850 nm. A Shimadzu UV-3101 PC spectrophotometer was used to record the optical absorption spectra. The current densityvoltage J-V characteristics were measured under illumination of a 15 mW/cm2 halogen lamp using an HP 1415B semiconductor parameter analyzer. Results The dark conductivity of the n-C60 layer D1 was measured just after its deposition to prevent the oxygen doping, while the p-ZnPc layer was exposed to air to induce the oxygen doping before we measure its dark conductivity D2 . Although the oxygen doping signicantly reduces the D1 value, it was reported that the doping process lasts longer than the time needed to take our measurements.21 In contrast, a short exposure to air initiates a</p> <p>z</p> <p>E-mail:</p> <p>JOURNAL OF APPLIED PHYSICS 98, 054311 2006</p> <p>Charge transport in hydrogenated boron-doped nanocrystalline silicon-silicon carbide alloysSeung Yeop Myong,a Oleg Shevaleevskiy, and Koeng Su LimDepartment of Electrical Engineering and Computer Science, Korea Advanced Institute of Science and Technology (KAIST), 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea</p> <p>Shinsuke Miyajima and Makoto KonagaiDepartment of Physical Electronics, Tokyo Institute of Technology (TIT), 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan</p> <p>Received 25 January 2005; accepted 28 July 2005; published online 9 September 2005 We have investigated the carrier transport mechanism of mixed-phased hydrogenated boron-doped nanocrystalline siliconsilicon carbide alloy p-nc-Si-SiC: H lms. From temperature-dependent dark conductivity measurements, we found that the p-nc-Si-SiC: H alloys have two different carrier transport mechanisms: one is the thermally activated hopping between neighboring crystallites near the room-temperature region and the other is the band tail hopping below 150 K. 2005 American Institute of Physics. DOI: 10.1063/1.2037871I. INTRODUCTION</p> <p>Thin lms of hydrogenated amorphous silicon aSi: H and silicon carbide a-SiC: H have attracted considerable research interest mainly due to their potential applications in electronics, optical devices, and window layers in photovoltaic thin-lm solar cells. To achieve a highefciency thin-lm solar cell, a window layer should have a high electrical conductivity and a wide optical band gap. Due to the incorporation of carbon atoms, a-SiC: H has a wider band gap than a-Si: H. However, the incorporated carbon atoms limit the electrical conductivity of a-SiC: H. We can improve the electrical conductivity of a-SiC: H by an impurity doping. However, the impurity doping reduces the optical band gap of lms. In recent years, one promising way to incorporate a wide band gap and a high conductivity has been proposed by producing the mixedc or phase structure consisting of microcrystalline nanocrystalline nc- Si grains embedded in a-Si: H network. We rstly reported on the preparation of hydrogenated boron B -doped nc-SiSiC:H p-nc-Si SiC: H alloy lms containing nc-Si grains embedded in a-SiC: H matrix via the photodecomposition of C2H4.1 Its optical transmittance is mainly governed by the a-SiC: H matrix, while nc-Si grains are responsible for the effective transport of charge carriers.2 This p-nc-Si SiC: H alloy has a higher electrical conductivity, optical transmittivity, carrier mobility, and doping efciency than the conventional undiluted p-a-SiC: H. Based on the deposition of p-nc-Si SiC: H alloy,14 H2-diluted p-a-SiC: H buffer layers of p-i-n-type a-Si: H or protocrystalline silicon pc-Si:H solar cells were prepared.58 We found that the natural hydrogen treatmentetching the defective undiluted p-a-SiC: H window layer and improving order in the window layertakes place just before the highly conductive, low absorption, and well-ordered H2-diluted p-a-SiC: H buffer layer deposition onto the undiluteda</p> <p>p-a-SiC: H window layer.6 Due to the natural hydrogen treatment, we can effectively reduce the recombination at the p / i interface, resulting in dramatic improvement of all solar cell parameters. The B doping of nc-SiSiC:H alloys considerably enhances their dark conductivity D . At the same time, the doping level may inuence the degree of the structural disorder, lm crystallinity, and defect density distribution. Various carrier transport mechanisms have been reported for aSi: H,911 c-Si: H,12,13 and nc-C:H lms.14 In this paper, we investigated the electric transport of p-nc-Si SiC: H alloys.II. EXPERIMENT</p> <p>FAX:</p> <p>82-42-869-8530; electronic mail:</p> <p>Films were deposited by the Hg-sensitized photoassisted chemical-vapor deposition photo-CVD technique using the mixture of SiH4, H2, B2H6, and C2H4 reactant gases. A lowpressure Hg lamp with resonance lines of 184.9 and 253.7nm was used as an UV light source to dissociate the mixture gases. In all depositions, the hydrogen dilution ratio H2 / SiH4 , ethylene gas ow ratio C2H4 / SiH4 , chamber pressure, substrate temperature, and Hg bath temperature were kept at 20, 0.07, 0.46 Torr, at 250 and 20 C, respectively. We deposited about 150170-nm-thick lms on Corning 7059 glass substrates with varying boron doping ratio B2H6 / SiH4 from 1000 to 8000 ppm. We performed Raman spectroscopy and dynamic force microscopy DFM to inspect the structural change of the lms. Raman spectra were measured by using JASCO Corp., NRS-1000 system. The wavelength of Ar laser is 532 nm. We used phase-modulated spectroscopic ellipsometer Jobin Yvon, UNISEL in order to measure the lm thickness and absorption coefcient. We measured the direct current dc D via coplanar Al contacts gap:1 mm formed by thermal evaporation. The temperature dependence of D was measured using a closed-cycle He cryostat with a proportionalintegral-derivative PID temperature controller. Since D measurements performed in a coplanar conguration are sensitive to the presence of surface adsorbates,15 we maintained</p> <p>Downloaded 24 May 2007 to Redistribution subject to AIP license or copyright, see</p> <p>JOURNAL OF APPLIED PHYSICS 99, 033520 2006</p> <p>A cubic phase of C3N4 synthesized in the diamond-anvil cellL. C. Minga and P. ZininHawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Honolulu, Hawaii, 96822</p> <p>Y. MengHPCAT, APS, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439</p> <p>X. R. Liu and S. M. HongLaboratory of High Pressure Physics, Southwest Jiaotong University, Chengdu, Sichuan, China 610031</p> <p>Y. XieStructure Research Laboratory and Department of Chemistry, University of Science and Technology of China, Hefei, Anhui, China 230026</p> <p>Received 1 September 2005; accepted 15 December 2005; published online 14 February 2006 A cubic phase of C3N4 was discovered. It was recovered at ambient conditions from the graphite-like C3N4 g-C3N4 phase subjected to pressures between 21 and 38 GPa in a diamond-anvil cell, laser heated to temperatures between 1600 and 3000 K. The x-ray-diffraction data of the phase are best explained by a cubic unit cell with the lattice parameters a = 3.878 0.001 . With an assumption of 1 molecule/ unit cell Z = 1 for the cubic phase, the molar volume of the cubic phase is 35.126 cm3 / mol and the density is 2.62 g / cm3. The density of the cubic phase is less than that which was predicted for the high-pressure phases but is 12% denser than the low-pressure graphitic phase = 2.336 g / cm3 . The cubic phase has not been predicted theoretically and represents an unknown structure in C3N4. 2006 American Institute of Physics. DOI: 10.1063/1.2168567I. INTRODUCTION II. EXPERIMENTAL METHODS</p> <p>The prediction by Liu and Cohen1 of the existence of a -C3N4 phase with a bulk modulus and hardness similar to diamond has fostered signicant research efforts to synthesize this material see review papers24 Various reports claim the synthesis of carbon nitride C3N4: however, none of the studies provide a comprehensive characterization of a single phase material.3 The data on the synthesis of dense C3N4 phases reported by different authors to date have yet to present unambiguous evidence for the crystallization of carbon nitrides with the proposed structures.4 It was also argued from chemical considerations that extensive network with CN single bonding has never been documented and only very local CN bonds are known as in amino acids.5 Recent studies of the phase transformation of the turbostratic carbon nitride t-CN under high pressure and temperature showed that at 4.7 GPa, the thermal decomposition of t-CN starts at 990 K and forms a disordered graphite. With a pressure increase up to 17.8 GPa, the onset temperature of the decomposition increases to 1850 K, and the process is accompanied by the formation of diamond.6 It was concluded that the low-compressibility forms of carbon nitride could not be obtained in a large volume press at pressure...</p>